Transcutaneous energy transfer (TET) systems are capable of transferring power across the skin without direct wire connections. This alleviates problems associated with infection at wire exit sites. A TET system will have a primary circuit including an inductive primary coil, located outside the body. The primary circuit generates a magnetic field. A secondary circuit, located inside the body, will include a secondary coil which couples with the magnetic field and the energy available is converted into power for the implanted device.
Because the coupling between the primary and secondary coils is usually low, a resonant circuit is implemented to provide power (voltage or current) amplification. A resonant circuit will have a natural resonant frequency which is dependent on the values of the capacitive and inductive components. For maximum power transfer, the resonant frequency of the secondary circuit should closely match the resonant frequency of the primary circuit.
Traditionally, the amount of power delivered to the implanted device is controlled by adjusting the strength of the magnetic field generated by the primary. An alternative approach is to adjust the resonant frequency of either the primary or the secondary away from the natural resonant frequency of the other part of the system. A miss-match in resonant frequencies will reduce the power delivered to the implanted device.
A mechanism for adjusting the reactive component (either resonant capacitor or resonant inductor) has been presented previously (in published New Zealand patent specifications NZ535012 and NZ526115) for the purpose of controlling power delivered to an implanted device. The method was based on switching a reactive component in and out of the circuit at the same rate as the resonant frequency to create an equivalent reactive element whose value is proportional to the phase of switching. Under this scheme, regulating the power requires switching at each cycle of the resonant frequency which creates power losses. Furthermore, the magnitude of the change in frequency that can be achieved is restricted by the harmonics introduced by the switching processes. This typically means that the value of the added effective capacitance is only a small proportion of the value of the fixed resonant capacitor, for example 20%.